In recent years, along with the remarkable development of portable instruments such as personal computers and portable telephones, batteries used to supply power to such portable instruments have been in great demand. In a battery used in such applications, a liquid electrolyte (electrolytic solution) utilizing a flammable organic solvent (dilution solvent) has been used as a medium through which ions move. A battery that utilizes such an electrolytic solution may ignite or explode due to leakage of the electrolytic solution, for example.
In order to solve such a problem, an all-solid-state battery that utilizes a solid electrolyte instead of a liquid electrolyte and is entirely formed of solid elements has been developed in order to ensure safety. Since the electrolyte of the all-solid-state battery is formed of a sintered ceramic (solid), leakage or ignition does not occur. Moreover, a deterioration in battery performance due to corrosion rarely occurs, for example. In particular, an all-solid-state lithium secondary battery has been extensively studied as a secondary battery of which the energy density can be easily increased (see Patent Document 1, for example).
The all-solid-state battery exhibits excellent safety and the like. On the other hand, since the entire electrolyte is solid, an improvement of ion conductivity of the solid electrolyte, a reduction of contact resistance (grain boundary resistance) between the electrolyte particles, and a reduction of contact resistance at the interface between the electrode and the electrolyte have been desired.
For example, since a liquid lithium ion secondary battery utilizes a liquid electrolyte, the space between the particles of the solid electrode and electrolyte is filled with the liquid electrolyte easily. Therefore, the contact area between the solid electrode and the liquid electrolyte does not correspond to the surface area of the solid electrode, but corresponds to the specific surface area of the solid electrode. The solid electrode advantageously comes in contact with the electrolyte when the solid electrode is sufficiently filled with the electrolyte.
Since the electrode and the electrolyte of the all-solid-state battery are solid, the contact area between the solid electrode and the electrolyte depends on the contact area between the particles of the solid electrode and the particles of the electrolyte. When the electrode and the electrolyte are not sufficiently sintered due to a low firing temperature, the particles of the solid electrode come in point-contact with the particles of the electrolyte. When the electrode and the electrolyte are sufficiently sintered so that the particles are fusion-bonded, the contact area between the particles increases so that the contact resistance (grain boundary resistance) at the interface decreases. Specifically, the contact resistance decreases as the contact area (necking) between the particles increases. However, since the reactivity of the materials must be taken into consideration when employing a firing temperature at which sufficient necking occurs, a substantial contact area cannot be easily obtained.
When producing an all-solid-state battery by way of trial, an electrode material (e.g., active material precursor) is applied to the flat surface of a solid electrolyte and is fired to form an electrode. In this case, the contact area does not exceed the area of the surface on which the electrode is formed. Since the contact area is the total area in which the particles of the electrode come in contact with the particles of the solid electrolyte, the contact area is generally smaller than the total surface area of the electrode.
In order to reduce the contact resistance (grain boundary resistance) at the interface between the electrode and the solid electrolyte, an all-solid-state battery in which a solid electrolyte is interposed between particles of an active material used for positive and negative electrodes has been disclosed (see Patent Document 2, for example). Specifically, the positive electrode and the negative electrode are formed by firing a green sheet obtained by forming a slurry prepared by mixing an active material and an electrolyte in the shape of a sheet, and the solid electrolyte (solid electrolyte layer) disposed between the electrodes is formed by firing a sheet formed only of a solid electrolyte material. The positive electrode, the solid electrolyte layer, and the negative electrode thus produced are press-bonded or fired under pressure to produce an all-solid-state battery. Such an all-solid-state battery is considered to allow an electrolyte network to be formed in the active material of the positive and negative electrodes.
Patent Document 1: JP-A-5-205741
Patent Document 2: JP-A-2000-311710